In a cellular communication system, one base station performs radio communication with a plurality of communication terminals simultaneously. In such a cellular communication system, there is a demand for transmission efficiency to be increased.
As a method of increasing the transmission efficiency over the downlink from a base station to a communication terminal, a method is proposed of performing scheduling that for allocating communication resources to communication terminals by means of time division, and of further setting a transmission rate for each communication terminal according to communication quality to transmit data. Hereinafter, this method is referred to as “adaptive modulation communication”.
Adaptive modulation communication will be described below using FIG. 1. In FIG. 1, it is assumed that a base station 11 is currently performing communication with communication terminals 12 through 14, which are within the cell area 15 covered by this base station 11. Communication terminals 20 through 22 are within the range of the cell area 15, but perform communication with a base station (not shown) other than this base station 11.
First, the base station 11 transmits a pilot signal to communication terminals 12 through 14. Each of communication terminals 12 through 14 estimates communication quality according to a CIR (Carrier to Interference Ratio) etc., using the pilot signal transmitted from the base station 11, and calculates a transmission rate at which communication is possible. Also, based on the transmission rate at which communication is possible, each of communication terminals 12 through 14 selects a communication mode indicating a combination of packet length, error correction, and modulation method, and transmits a signal indicating the communication mode to the base station 11.
Based on the communication mode selected by each of communication terminals 12 through 14, the base station 11 performs scheduling, sets a transmission rate for each communication terminal, and notifies a signal indicating communication resource allocation to each of communication terminals 12 through 14 via a control channel.
The base station 11 transmits data only to the relevant communication terminal in its assigned time via a data channel. For example, when time t1 is assigned to communication terminal 12, in time t1 the base station 11 transmits data only to communication terminal 12, and does not transmit to communication terminals 13 and 14. Also, transmission power when the base station 11 transmits data to communication terminals 12 through 14 is always constant.
Parallel to adaptive modulation communication, ordinary CDMA (Code Division Multiple Access) communication is performed in parallel between the base station 11 and communication terminals 12 through 14 in a different band from that for adaptive modulation communication.
However, in above-described conventional adaptive modulation communications, the following problem arises. Referring again to FIG. 1, the base station 11 transmits data to each of communication terminals 12 through 14 always using fixed power, regardless of the distances to communication terminals 12 through 14. This power is high enough to ensure that reception quality is sufficiently good at all communication terminals in the cell area 15.
Consequently, there is a possibility that, among communication terminals performing adaptive modulation communication with a base station other than base station 11 (hereinafter referred to as “another base station”), communication terminals within the cell area 15 covered by base station 11 (in FIG. 1, communication terminals 20 through 22) may receive interference due to a signal transmitted to any one of communication terminals 12 through 14 from base station 11. As a result, the communication quality of a communication terminal receiving interference in this way will deteriorate.
For example, if the time when base station 11 transmits data to communication terminal 12 via a data channel is coincident with the time when another base station transmits data to communication terminal 20 via a data channel, communication terminal 20 receives interference due to the signal transmitted from base station 11 to communication terminal 12.
Also, if base station 11 transmits adaptively modulated signals to a plurality of communication terminals (for example, communication terminals 12 through 14) at the same time, the communication quality of that plurality of communication terminals will deteriorate because delayed waves of the signals transmitted to that plurality of communication terminals will cause mutual interference.
As explained above, in above-described conventional adaptive modulation communications there is a problem in that an adaptively modulated signal transmitted from a base station causes interference with a communication terminal performing adaptive modulation communication with another base station, or with a communication terminal performing communication with the same base station at the same time.